An optical device using a dielectric substrate of an electrooptical crystal such as lithium niobate (LiNbO3) and lithium tantalite (LiTaO3) is manufactured by forming an optical waveguide by forming a metal film of titanium (Ti) on a part of the crystal substrate and causing heat diffusion, or by causing proton exchange in benzoic acid after patterning, and then by arranging electrodes near the optical waveguide.
In such an optical device, an optical waveguide has, for example, a branching unit that branches a light beam, parallel waveguides that respectively transmit each branch of light beam, and a combining unit that combines the respective branches of the light beam transmitted by the parallel waveguides. Further, a signal electrode and a ground electrode are provided along the parallel waveguides, and a coplanar electrode is formed. When a Z-cut substrate is used as the dielectric substrate, a signal electrode is arranged on the optical waveguide along the waveguide, to utilize a change of refractive index caused by an electric field in the Z direction.
When the optical modulator is driven at a high speed, ends of the signal electrode and the ground electrode are connected through a resistor to form a traveling-wave electrode, and a microwave signal is applied from an input side of the signal electrode. Thus, the respective refractive indexes of the parallel waveguides change as +Δna and −Δnb by the electric field, and the phase difference between the parallel waveguides changes. Interference of each light beam at the combining unit is Mach-Zehnder interference, and an intensity-modulated optical signal is output from the combining unit.
Two units of such optical modulators are connected in series, and one of them is driven by a clock signal, and the other is driven by a non return to zero (NRZ) data signal, thereby generating a return to zero (RZ) signal. Moreover, a configuration in which two modulators are integrated on a signal chip using a folded waveguide has been proposed. At a part of the folded waveguide, by arranging a groove along an outer periphery of a U-shaped waveguide, loss of light can be suppressed (for example, Japanese Laid-Open Patent Publication No. 2004-287093).
Dielectric substrates using LiNbO3 or LiTaO3 have considerably high pyroelectric effect. Accordingly, a pyroelectric charge is generated when the temperature of a dielectric substrate changes, and the surface of the dielectric substrate is charged to generate a high electric potential. Therefore, when a metal pattern of Ti or the like is diffused on the surface of the dielectric substrate by applying heat, the large amount of pyroelectric charge generated is likely to be built up in the metal pattern (waveguide pattern). As a result, electrical discharge occurs in the metal pattern, breaking the optical waveguide pattern.
As a measure against this problem, a technique is disclosed in which an end of a waveguide pattern to be an optical waveguide is connected to a conductor pattern having a relatively large area (for example, International Publication Pamphlet No. 94/010592). With such a configuration, pyroelectric charge built up in the waveguide pattern escapes to the conductor pattern, thereby suppressing damage of the optical waveguide by electrical discharge.
However, in the conventional technique described above, if the optical waveguide has a folded part, the folded part and the conductor pattern to let the pyroelectric charges escape are distanced. Therefore, when a heat diffusion processing is performed, charge density differs between the folded part and the conductor pattern, and an electric potential difference is generated between the folded part and the conductor pattern. When this difference becomes large, electrical discharge occurs between the folded part and the conductor pattern, and as a result, a part of the optical waveguide around the folded part is damaged.